The present invention relates to a thermal protector built into a transformer, a motor, or the like. More particularly, it relates to a self-holding type thermal protector that incorporates a heating resistor consisting of a PTC (Positive Temperature Coefficient) element to maintain the open state by means of heat generated in the heating resistor.
There has been proposed a thermal protector that incorporates a PTC element such as a positive thermistor as a heating resistor to maintain the open state by means of heat generated in the heating resistor.
FIG. 12 shows a thermal protector of this type proposed in Japanese Patent Provisional Publication No. 7-282701.
In this thermal protector, a load current flows in the order of a terminal a for external connection, a bimetal b, a movable contact c, a fixed contact d, a fixed electrode e, and a terminal f for external connection or in the order reverse to the above.
When the temperature of the bimetal b is increased to a predetermined operating temperature by overcurrent or heat applied from a load, the bimetal b performs a reversing operation, so that the movable contact c separates from the fixed contact d, by which the current flowing to the load is interrupted.
Between the proximal portion of the bimetal b and the fixed electrode e, a positive thermistor g, which is a PTC element, is disposed. When the movable contact c separates from the fixed contact d, the voltage across the terminals a and f is applied to the thermistor g, so that the thermistor g becomes in an energized state to generate heat.
When the thermistor g generates heat, the heat is transmitted to the bimetal b, whereby the reversed state of the bimetal b is maintained.
The above-described conventional thermal protector, in which the lower face of the proximal end portion of the bimetal b is in direct contact with the thermistor g, offers an advantage that the heat generated by the thermistor g is transmitted efficiently to the bimetal b. However, it has the disadvantages described below.
The face on the highly expanded side (lower face) of the bimetal b has a bad surface condition due to oxidation etc., and also has a high electrical resistance. In the conventional thermal protector, in which the face on the highly expanded side of the bimetal b is in contact with the electrode face of the thermistor g, the stability of electrical contact of the bimetal b with the thermistor g is insufficient. Therefore, there is a possibility that a proper heating current does not flow in the thermistor g after the bimetal b is reversed.
The present invention has been made in view of the above situation, and accordingly an object thereof is to provide a thermal protector in which the electrical conductivity of a heating resistor consisting of a PTC element is not impaired, and the heat generated in the heating resistor can be transmitted efficiently to a bimetal.
To achieve the above object, the present invention provides a thermal protector comprising: a conductive fixed plate having a fixed contact at one end and formed with a first terminal for external connection at the other end; a movable plate having elasticity and conductivity provided with a movable contact at one end, the movable contact being brought into contact with the fixed contact by the elastic force; a second terminal for external connection connected to the movable plate; a bimetal performing a reversing operation when the temperature thereof exceeds a predetermined value, whereby the movable plate is displaced by the reversing force of the bimetal to separate the movable contact from the fixed contact; and a heating resistor, consisting of a PTC element, interposed between the fixed plate and the movable plate, wherein the central portion of the movable plate is cut into a U shape to form a tongue provided along the longitudinal axis of the movable plate, the tongue is brought into close contact with an upper electrode face of the heating resistor, a lower electrode face of the resistor is brought into contact with the fixed plate, and portions of the movable plate located on both sides of the cut operate as an elastic arm.
In an embodiment of the present invention, the bimetal is arranged in parallel above the movable plate so that one end portion thereof is engaged with the front end of the movable plate and the other end portion thereof is engaged above the tongue, and the front end height of the movable plate in a state in which the bimetal is not reversed is set at a height such that a part of the bimetal can be brought into contact with the upper face of the tongue when the bimetal is reversed.
In an embodiment of the present invention, the front end height of the movable plate in a state in which the bimetal is not reversed is set not higher than the height of the upper electrode face of the heating resistor.
In an embodiment of the present invention, a protrusion serving as a reversing fulcrum for the bimetal is provided on the tongue.
In an embodiment of the present invention, an elastic raised portion with a wavelike cross section is formed at a portion contacting with the lower electrode face of the heating resistor on the fixed plate so that the raised portion is elastically brought into contact with the lower electrode face.
In an embodiment of the present invention, a plurality of the raised portions are formed in the longitudinal direction of the fixed plate, and the height of the lower end of a trough portion formed between the raised portions is set not lower than the upper face height of a flat portion of the fixed plate.
In an embodiment of the present invention, the lower end of the trough portion is formed flat.
In an embodiment of the present invention, the contact surface of the tongue with the upper electrode face of the heating resistor and the contact surface of the fixed plate with the lower electrode face of the heating resistor are subjected to surface treatment to improve the electrical contact stability.
In an embodiment of the present invention, conductive paste is interposed between the upper electrode face of the heating resistor and the tongue and between the lower electrode face of the heating resistor and the fixed plate to improve the electrical contact stability.
In an embodiment of the present invention, in a first mode of the invention, the bimetal is arranged in parallel above the movable plate so that one end portion thereof is engaged with the front end of the movable plate and the other end portion thereof is engaged above the tongue, and a protrusion brought into contact with the reversed bimetal is provided on the tongue.
The present invention achieves the following effects.
(1) A new part for incorporating the heating resistor consisting of a PTC element need not be added, so that the thermal protector can be constructed with an equal number of parts to the number of parts for the conventional thermal protector using the movable plate.
(2) Since electricity is conducted to the heating resistor via the movable plate, the electrical conductivity of the heating resistor is improved.
(3) Since the heating resistor is carried by a part of the movable plate to which the bimetal is attached, there is provided an advantage on the heat transfer surface that the heating efficiency of the bimetal is increased.
(4) Since the narrow arm portions are formed on both sides of the tongue provided on the movable plate, the movable plate is displaced easily. Therefore, a load at the time when the bimetal is reversed is reduced, so that the operating characteristics of the bimetal are stabilized.
(5) When the bimetal is reversed, the bimetal can be brought into contact with the tongue of the movable plate in close contact with the heating resistor, so that the heat generated in the heating resistor is transmitted efficiently to the bimetal. As a result, the reversed state of the bimetal can be held stably.
(6) Since the elastic raised portion with a wavelike cross section is formed on the fixed plate, any dimensional error of the heating resistor is absorbed, so that the upper electrode face of the heating resistor can be pressed on the lower face of the tongue of the movable plate, which is a reference plane. Therefore, the electrical contact stability of the heating resistor is improved. Also, since the contact area of the fixed plate with the heating resistor decreases, the outflow of heating energy of the heating resistor to the fixed plate is restrained to the utmost. As a result, a loss of heating energy of the heating resistor is reduced.